This invention relates to an erasable optical data storage medium and particularly to such a medium including an integral recording layer having zones of different optical and thermal-mechanical properties, and to a method for constructing such a medium.
Within the past decade there has been tremendous interest and developmental work in the area of optical data recording and storage techniques, primarily because of the significant advantage such technology offers over magnetic data technology including storage capacity and archival life. One of the reasons optical data technology has not yet supplanted magnetic data recording and storage techniques is the lack of an effective, reliable, and economical optical data storage medium which is also erasable.
Substantially all of the optical data storage mediums involve multi-layer structures. However, not all of the layers of the medium are actually involved in actively recording or erasing data in the medium. Most mediums include non-active layers as supporting substrates or protective overlays. While some optical data storage mediums have a single active or recording layer, other mediums have two or more active or recording layers.
It is elementary that the structure of the medium is related to the method which is used for recording and erasing data in that medium. For example, a medium having a dual active recording layer might typically have one metallic layer and one organic layer. A method associated with such a medium would involve heating the metal layer until the organic layer adjacent thereto evaporates, forming either an optically detectable crater (ablative method) or an optically detectable bubble (vesicular method) in the medium. Mediums of the type described above are disclosed in the following: Cornet, U.S. Pat. Nos. 4,404,656, 4,577,291, 4,398,203.
A related medium might use a dyed, light-absorbtive layer rather than a metal layer to absorb heat from the laser and form ablative deformations, such as craters, or vesicular deformations, such as bubbles, in the medium. Mediums of this type are disclosed in Maffit U.S. Pat. No. 4,430,659 and Bell U.S. Pat. Nos. 4,285,056 and 4,300,227.
A pending patent application filed by applicant's assignee discloses a structure having a dual recording layer for use with a non-ablative, non-vesicular technique. Pursuant to MPEP 608.01(p) commonly owned U.S. patent application Ser. No. 525,813 is incorporated herein by reference with particular attention drawn to the description of the prior art contained therein, the method disclosed therein for recording and erasing data, and to the system described therein for recording, reading and erasing data. The medium disclosed in the commonly owned prior application employed two discrete polymer layers, an expansion layer and an overlying retention layer, each layer having different optical and thermal-mechanical properties. The method associated with this medium involves selectively heating one or the other of the layers and employing the different thermal-mechanical properties of the two layers to selectively record or erase data in the form of optically detectable deformations in the medium. More particularly, to record, the heated expansion layer pushes on and viscoelastically deforms the retention layer thereby creating an optically detectable and reversible deformation which is held in place by the retention layer. To erase, the retention layer is heated above its glass transition temperature and is pulled back to its original conformation by the elastically strained expansion layer.
With respect to mediums having a single active layer, an ablative technique is often used to burn away a portion of the active layer to form an optically detectable pit or crater in the active layer as disclosed in Howe U.S. Pat. Nos. 4,336,545 and 4,360,908. Other methods associated with mediums having single recording layers involve changing the optical density of the active layer such as disclosed in Willis U.S. Pat. No. 4,264,986 and Ohta U.S. Pat. No. 4,278,734.
The data storage mediums which are adapated to be used with ablative or vesicular techniques are generally not erasable since such methods involve heating a portion of the medium until it evaporates or changes state. Mediums which use two or more active or recording layers are generally more difficult and expensive to build and have the inherent problem of ensuring that the active layers remain bonded to each other during the recording and erasing steps.